Apparatuses for transforming thermal energy directly into electric energy using a minimum of moving parts have been known since long.
At the end of the 19′th century, Edison and Tesla described devices based on thermomagnetic materials for converting thermal energy into electricity. Edison's pyro-magnetic generator as described in U.S. Pat. No. 380,100 includes a thermomagnetic working material, means for magnetizing the working material, sources of heat and cold connected to the working material, and a winding enclosing the working material and in which an alternating electric current is induced by thermally cycling the working material. Tesla discloses in U.S. Pat. No. 428,057 some improvements of Edison's generator by suggesting an alternative heat exchanging mechanism.
Chilowsky discloses in U.S. Pat No. 2,510,801 a device for thermomagnetic energy conversion, wherein the temperature variations are achieved by hot and cold fluids in a closed fluid circuit.
Bartels discloses in German patent No. 23 47 377 a device similar to Edison's generator, but suggests gadolinium as magnetic working medium. In one embodiment a permanent magnet is provided to create the magnetic flux in the thermomagnetic material and in another embodiment a battery is used to induce a current in a coil to thereby form an electromagnet to create the magnetic flux.
While the apparatuses described above do convert thermal energy directly to electric energy, they seem not to be efficient, compact, optimized for practical use, and/or cost-efficient to manufacture.